"The More You Magnify Something...

Bill Ferris said:

the dimmer it becomes."

This statement was recently made in a "Beginner's Questions" forum thread. That thread has been locked but the conmment merits response because, in many situations, magnifying something does not make it dimmer. And this does have relevance to photography.

Click to expand...

Magnifying something visible is spreading the same number of photons over a larger area, so yes it always makes it dimmer. In photography (& microscopy etc) we can add more light, increase the aperture (not necessarily f/number), or expose for longer to compensate but this is actually a second stage not just magnification.

Bill Ferris said:

Let's start by clarifying a couple of terms. "Something" refers to a thing in its entirety. If the something, for example is a 6-foot tall person, then we're talking about the entire 6-foot tall person. A part of that person (e.g. their head hand or leg) is a different something.

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Assuming the 'something' is uniformly bright the distinction is irrelevant.

Bill Ferris said:

"Dimmer" refers to the brightness of the something. In photography and life we can describe brightness in a number of ways. The two I'm using in this thread are the total volume of light received from the thing and the brightness per unit area of the thing. To make something appear dimmer, we're making it appear less bright either in total volume of light received from the object out the object's brightness per unit area; its surface brightness.

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I would say 'Volume of light' is meaningless, light does not have volume.

Bill Ferris said:

"Magnify" refers to making a thing appear larger in size. This can be accomplished in at least a couple of ways.

One way to magnify something is to move either yourself or the thing nearer. A 6-foot tall person who appears only 1-foot tall to the unaided eye can be made to look 2-feet tall by halving the distance between you and the other person.

Imagine accomplishing this by walking along a path toward that person. You're both outside and evenly illuminated in bright sunlight. With each step closer, the person appears a bit larger. You are effectively magnifying them. And according to the inverse-square law, when you've halved the distance to the person, 4-times as much light from that person is being delivered to your eyes.

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I wouldn't consider this as magnifying. Outside of macro work where magnification is defined as image size/object size, its normal to refer to magnification as image size through the optics/image size to the human eye (roughly with a 50mm eqivalent lens)

If you do the degree of dimming indeed becomes insignificant, but the lens needs to be further away to focus closer & this does dim the image even if it's not noticeably so till you get to macro work.

Bill Ferris said:

Now, your eye pupils automatically dilate to manage the brightness of objects in your field of view. So, let's assume your eye pupils will adjust to maintain a constant surface brightness for the person you are approaching. In that sense of brightness, nothing changes. We can accurately say the person gets neither darker nor brighter. Their apparent surface brightness is constant. They do not get dimmer.

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If your pupils adjust that's changing your aperture, a secondary function that compensates for the dimming

Bill Ferris said:

However, they do get larger. As a result of maintaining a constant surface brightness over an increasing area, the total volume of light delivered to and captured by your eyes increases. In fact, 4-times as much light is needed to maintain a constant surface brightness of an object which is twice as large in size. Being twice as large in size means the object has 4-times the surface area.

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The surface area is not increasing, just the apparent surface area. The same number of photons will be hitting the subjects surface whatever it's apparent area.

I suppose the inverse square law would be working in your favour, photons that would have just missed your lens are now captured, but as above this simply getting closer isn't normally seen as magnification.

Bill Ferris said:

In this sense, the something (a 6-foot tall person) which has been magnified is actually brighter than before. Not only has this magnified something not been made dimmer, it's been made brighter.

How does this apply to photography? Suppose you're making a portrait of a friend and you've got the f-stop, shutter speed and ISO dialed in for a perfect exposure. The only problem is, the subject looks too small in the frame. If you move nearer to your subject so they're filling more of the frame, they will not become dimmer. As long as the light level in the space is constant, your friend will have the same surface brightness despite the fact you are actually collecting more total light from them. You will not need to change your settings to get a good exposure.

In photography, we can magnify something by using a longer focal length lens. For example, suppose you are using a 70-200mm f/2.8 zoom lens to make the portrait of your friend. You started at 70mm f/2.8 but didn't like how small your friend was in the frame. So, you moved nearer to make your friend appear larger and fill more of the frame. You like their apparent size but, now, your not pleased with the way their facial features look a bit distorted from that perspective.

So, you go back to where you started and zoom from 70mm to 200mm. Your friend fills the frame, again, which you like. And from this perspective, their facial features look normal to you. The lens maintains a constant f/2.8 throughout the zoom range by increasing the aperture as focal length increases. As a result of capturing more light at the longer focal length, your friend's surface brightness remains constant. Again, you're collecting more total light from your subject, maintaining a constant subject surface brightness and are able to use the same exposure settings.

Click to expand...

A 70mm/2.8 has an aperture 25mm wide, the 200mm/2.8 has an aperture 71.4mm wide (both as seen from the entrance pupil). They may be the same f/number but the aperture is a very different size. You are changing variable beyond simply magnification.

Bill Ferris said:

So far, we've examined two different ways of magnifying something, neither of which causes that something to be dimmer. In fact, in both scenarios, we're actually collecting more total light from the subject and maintaining a constant surface brightness.

Of course there are scenarios in which we can magnify an object and make it appear dimmer. Suppose you are looking at the full Moon through a telescope and using an eyepiece providing a low power view. In other words, you see the full moon and a substantial amount of sky surrounding it. Next, you put a higher magnification eyepiece into the telescope and see the Moon filling the entire field of view.

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With my interchangeable eyepiece telescopes I don't think I have an eyepiece that will get the whole moon in!

Bill Ferris said:

We've not moved closer to the Moon and we've not increased the aperture used to collect light from the moon. As a result at the higher magnification view, we're still collecting the same total volume of light from the Moon. In that context, the Moon's brightness is constant. However, that same total volume of light is spread across a larger surface area. As a result, the Moon's surface brightness is reduced and this can be perceived as an object looking dimmer...less bright.

Click to expand...

You've hit on an example where you can't change other variables, unless you have very good friends at NASA or change the scope.

Bill Ferris said:

How does this apply to photography? If using a variable aperture zoom lens to make a portrait of a friend, zooming in to a longer focal length so your friend fills more of the frame may result in your friend having a lower surface brightness to the camera sensor. If you've chosen exposure settings that make a nice image at 70mm f/5, zooming to 200mm f/6.3 to improve the framing will require an adjustment either to shutter speed or ISO to produce an image in which your friend has the same lightness as in the 70mm f/5 photo.

Of course, there are also scenarios in which a combination of changes to subject distance, focal length and f-stop can result in an image in which the subject is magnified, less total light is collected from the subject and the subject displays a lower surface brightness. But this is not a guaranteed outcome. There are multiple scenarios in which we can magnify a subject, have it maintain a constant surface brightness and deliver even more total light to the sensor than when it was smaller in apparent size.

Just some food for thought.

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You say: "The lens maintains a constant f/2.8 throughout the zoom range by increasing the aperture as focal length increases."

Is this increase in aperture to maintain the same f stop the reason why longer focal length lenses have less depth of field than shorter focal length lenses? If anyone can explain this clearly, Bill, it is you.

petrochemist said:

Bill Ferris said:

the dimmer it becomes."

This statement was recently made in a "Beginner's Questions" forum thread. That thread has been locked but the conmment merits response because, in many situations, magnifying something does not make it dimmer. And this does have relevance to photography.

Click to expand...

Magnifying something visible is spreading the same number of photons over a larger area, so yes it always makes it dimmer.

Click to expand...

That's one scenario but not the only scenario.

petrochemist said:

In photography (& microscopy etc) we can add more light, increase the aperture (not necessarily f/number), or expose for longer to compensate but this is actually a second stage not just magnification.

Click to expand...

That's a narrow definition of magnification; one I do not accept. Magnification is the process of making something appear larger. If you look just at scenarios involving an optical system that magnifies, you must consider both focal length and aperture. And there is no requirement that the aperture remain constant. In photography where constant f-ratio zoom lenses alter the size of the entrance pupil as focal length changes, subjects can be magnified without becoming dimmer.

petrochemist said:

petrochemist said:

Let's start by clarifying a couple of terms. "Something" refers to a thing in its entirety. If the something, for example is a 6-foot tall person, then we're talking about the entire 6-foot tall person. A part of that person (e.g. their head hand or leg) is a different something.

Click to expand...

Assuming the 'something' is uniformly bright the distinction is irrelevant.

petrochemist said:

"Dimmer" refers to the brightness of the something. In photography and life we can describe brightness in a number of ways. The two I'm using in this thread are the total volume of light received from the thing and the brightness per unit area of the thing. To make something appear dimmer, we're making it appear less bright either in total volume of light received from the object out the object's brightness per unit area; its surface brightness.

Click to expand...

I would say 'Volume of light' is meaningless, light does not have volume.

Click to expand...

Volume of light is a phrase commonly used to describe the total light captured from an object. It is far from meaningless. The magnitude of an extended celestial object (e.g. a galaxy) is a description of the total light received from that object. Through the use of optical systems, we can capture even more light from that distant galaxy, making it appear even brighter in total magnitude at the eyepiece. In the context of this discussion, the total light captured from something during an exposure is central to understanding the question of whether or not the image of that thing focused on the sensor has changed brightness.

petrochemist said:

petrochemist said:

"Magnify" refers to making a thing appear larger in size. This can be accomplished in at least a couple of ways.

One way to magnify something is to move either yourself or the thing nearer. A 6-foot tall person who appears only 1-foot tall to the unaided eye can be made to look 2-feet tall by halving the distance between you and the other person.

Imagine accomplishing this by walking along a path toward that person. You're both outside and evenly illuminated in bright sunlight. With each step closer, the person appears a bit larger. You are effectively magnifying them. And according to the inverse-square law, when you've halved the distance to the person, 4-times as much light from that person is being delivered to your eyes.

Click to expand...

I wouldn't consider this as magnifying.

Click to expand...

Call it whatever you want. The subject appears larger.

petrochemist said:

petrochemist said:

Now, your eye pupils automatically dilate to manage the brightness of objects in your field of view. So, let's assume your eye pupils will adjust to maintain a constant surface brightness for the person you are approaching. In that sense of brightness, nothing changes. We can accurately say the person gets neither darker nor brighter. Their apparent surface brightness is constant. They do not get dimmer.

Click to expand...

If your pupils adjust that's changing your aperture, a secondary function that compensates for the dimming

Click to expand...

Actually, because you receive more total light as you get nearer the subject, it does not get dimmer. There's no need to increase eye pupil size to maintain subject surface brightness.

petrochemist said:

petrochemist said:

However, they do get larger. As a result of maintaining a constant surface brightness over an increasing area, the total volume of light delivered to and captured by your eyes increases. In fact, 4-times as much light is needed to maintain a constant surface brightness of an object which is twice as large in size. Being twice as large in size means the object has 4-times the surface area.

Click to expand...

The surface area is not increasing, just the apparent surface area. The same number of photons will be hitting the subjects surface whatever it's apparent area.

I suppose the inverse square law would be working in your favour, photons that would have just missed your lens are now captured, but as above this simply getting closer isn't normally seen as magnification.

Click to expand...

The subject is seen as becoming larger and is definitely not getting dimmer. We're gathering more light from the subject, that light covers a larger area, its surface brightness remains constant and, therefore the subject is not dimmer.

petrochemist said:

petrochemist said:

In this sense, the something (a 6-foot tall person) which has been magnified is actually brighter than before. Not only has this magnified something not been made dimmer, it's been made brighter.

How does this apply to photography? Suppose you're making a portrait of a friend and you've got the f-stop, shutter speed and ISO dialed in for a perfect exposure. The only problem is, the subject looks too small in the frame. If you move nearer to your subject so they're filling more of the frame, they will not become dimmer. As long as the light level in the space is constant, your friend will have the same surface brightness despite the fact you are actually collecting more total light from them. You will not need to change your settings to get a good exposure.

In photography, we can magnify something by using a longer focal length lens. For example, suppose you are using a 70-200mm f/2.8 zoom lens to make the portrait of your friend. You started at 70mm f/2.8 but didn't like how small your friend was in the frame. So, you moved nearer to make your friend appear larger and fill more of the frame. You like their apparent size but, now, your not pleased with the way their facial features look a bit distorted from that perspective.

So, you go back to where you started and zoom from 70mm to 200mm. Your friend fills the frame, again, which you like. And from this perspective, their facial features look normal to you. The lens maintains a constant f/2.8 throughout the zoom range by increasing the aperture as focal length increases. As a result of capturing more light at the longer focal length, your friend's surface brightness remains constant. Again, you're collecting more total light from your subject, maintaining a constant subject surface brightness and are able to use the same exposure settings.

Click to expand...

A 70mm/2.8 has an aperture 25mm wide, the 200mm/2.8 has an aperture 71.4mm wide (both as seen from the entrance pupil). They may be the same f/number but the aperture is a very different size. You are changing variable beyond simply magnification.

Click to expand...

In photography we use both distance from a subject and optical systems to control how large the subject appears within the frame. In the context of using a variable focal length lens's ro magnify a subject, this involves a focal length and an entrance pupil. There is no requirement that entrance pupil size remain constant as focal length changes and a subject appears larger. It is one scenario but the only scenario.

I do not accept the unnecessary restriction that magnification can only be understood within the context of an optical system with a fixed entrance pupil.

petrochemist said:

petrochemist said:

So far, we've examined two different ways of magnifying something, neither of which causes that something to be dimmer. In fact, in both scenarios, we're actually collecting more total light from the subject and maintaining a constant surface brightness.

Of course there are scenarios in which we can magnify an object and make it appear dimmer. Suppose you are looking at the full Moon through a telescope and using an eyepiece providing a low power view. In other words, you see the full moon and a substantial amount of sky surrounding it. Next, you put a higher magnification eyepiece into the telescope and see the Moon filling the entire field of view.

Click to expand...

With my interchangeable eyepiece telescopes I don't think I have an eyepiece that will get the whole moon in!

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The full Moon in my 18-inch Obsession is pretty impressive. Of course, it kills whatever dark adaptation you've achieved...not that that would be much on a full Moon night.

petrochemist said:

petrochemist said:

We've not moved closer to the Moon and we've not increased the aperture used to collect light from the moon. As a result at the higher magnification view, we're still collecting the same total volume of light from the Moon. In that context, the Moon's brightness is constant. However, that same total volume of light is spread across a larger surface area. As a result, the Moon's surface brightness is reduced and this can be perceived as an object looking dimmer...less bright.

Click to expand...

You've hit on an example where you can't change other variables, unless you have very good friends at NASA or change the scope.

petrochemist said:

How does this apply to photography? If using a variable aperture zoom lens to make a portrait of a friend, zooming in to a longer focal length so your friend fills more of the frame may result in your friend having a lower surface brightness to the camera sensor. If you've chosen exposure settings that make a nice image at 70mm f/5, zooming to 200mm f/6.3 to improve the framing will require an adjustment either to shutter speed or ISO to produce an image in which your friend has the same lightness as in the 70mm f/5 photo.

Of course, there are also scenarios in which a combination of changes to subject distance, focal length and f-stop can result in an image in which the subject is magnified, less total light is collected from the subject and the subject displays a lower surface brightness. But this is not a guaranteed outcome. There are multiple scenarios in which we can magnify a subject, have it maintain a constant surface brightness and deliver even more total light to the sensor than when it was smaller in apparent size.

Just some food for thought.

Click to expand...

Click to expand...

• The camera lens projects an image of the outside world on the surface of film or digital image sensor. This image consists of incalculable number of tiny juxtaposed circles. Each circle is the smallest fraction of an image that conveys intelligence. Under a microscope they appear circular with indistinct boundaries. These are called circles of confusion. When examining an image, we will proclaim that image sharp if these are seen as points not circles. If we can distinguish circles and not points, we perceive this image as compromised as to sharpness. The accepted size for sharpness is ½ mm viewed from normal reading distance (500mm).

• These tiny circles of confusion are molded from the working diameter of the camera’s iris diaphragm (aperture entry size). In other words, large working aperture diameters yield large circles and small working diameter apertures yield smaller circles of confusion.

• Now the f-number of a lens intertwines the focal length divided by the working diameter. Consider a 50mm lens operating at 12.5mm diameter. The f-number for this lens and setting is 50 ÷ 12.5 = 4 written as f/4. Whereas a 200mm lens with a working diameter of 50mm, operates at 200 ÷ 50 = 4 written as f/4. Both operate at f/4 but the longer lens has a larger aperture diameter.

• Since the diameter of the circle of confusion derived from aperture diameter, the shorter lens with its lesser working diameter projects smaller circles, it will have superior depth-of-field.

almostgreen said:

You say: "The lens maintains a constant f/2.8 throughout the zoom range by increasing the aperture as focal length increases."

Is this increase in aperture to maintain the same f stop the reason why longer focal length lenses have less depth of field than shorter focal length lenses? If anyone can explain this clearly, Bill, it is you.

Click to expand...

In simplest terms, if you are a fixed distance from a subject, depth of field becomes more shallow as the lens entrance pupil increases.

If one frames a shot at 50mm f/8 and focuses on a subject 10-feet in front of you, the image will have a certain depth of field. From that same location using thay same gear, open the lens aperture to f/5.6 and make a photo of the same subject. Open up to F4, f/2.8 and make photos. Compare the images. The images made at the smaller f-stops (larger entrance pupils) will have noticeably shallower depths of field.

If one replaces the 50mm lens with a 24-70mm f/2.8 zoom, sets the f-stop to f/2.8 and makes photos of the same subject at 24mm, 50mm and 70mm, the depths of field of the photos will be shallower as focal length increases. To maintain the f/2.8 focal ratio throughout the zoom range, lens aperture (entrance pupil) must increase as focal length increases. As entrance pupil increases, depth of field becomes more shallow when subject distance is keep constant.

The math revolving around depth-of-field formulas is a subset of the formula used to find the hyperfocal distance. We multiply the working diameter of the camera’s lens (aperture diameter in millimeters) by 1000. Why 1000? The diameter of the aperture (f-number) viewed from 1000 diameters distance will be seen as a super tiny dimensionless point. Thus 1000x the dimeter works nicely to derive the hyperfocal distance. This value might not be stringent enough for giant enlargements. These are often based on 1/2000 of the dimeter of the iris. Kodak often used 1/1750 and Leica uses 1/1500 for critical work. Depth-of-field calculators often use different sizes. The choice is a guesstimate based on the magnification required to make the display image and the distance the observer is from this image.

A Marcus said:

The math revolving around depth-of-field formulas is a subset of the formula used to find the hyperfocal distance. We multiply the working diameter of the camera’s lens (aperture diameter in millimeters) by 1000. Why 1000? The diameter of the aperture (f-number) viewed from 1000 diameters distance will be seen as a super tiny dimensionless point. Thus 1000x the dimeter works nicely to derive the hyperfocal distance. This value might not be stringent enough for giant enlargements. These are often based on 1/2000 of the dimeter of the iris. Kodak often used 1/1750 and Leica uses 1/1500 for critical work. Depth-of-field calculators often use different sizes. The choice is a guesstimate based on the magnification required to make the display image and the distance the observer is from this image.

Click to expand...

That's NOT the formula for determining the hyperfocal distance. It's a rule of thumb that might give a rough idea for a specific format.

To actually determine it you need to use the laws of optics & the circle of confusion appropriate for the sensors size & final output/viewing conditions.

A Marcus said:

The math revolving around depth-of-field formulas is a subset of the formula used to find the hyperfocal distance. We multiply the working diameter of the camera’s lens (aperture diameter in millimeters) by 1000. Why 1000? The diameter of the aperture (f-number) viewed from 1000 diameters distance will be seen as a super tiny dimensionless point. Thus 1000x the dimeter works nicely to derive the hyperfocal distance. This value might not be stringent enough for giant enlargements. These are often based on 1/2000 of the dimeter of the iris. Kodak often used 1/1750 and Leica uses 1/1500 for critical work. Depth-of-field calculators often use different sizes. The choice is a guesstimate based on the magnification required to make the display image and the distance the observer is from this image.

Click to expand...

This is completely confused. You're talking about the circle of confusion, and it isn't a fraction of 'the diameter of the iris;, it's the frame diagonal.

petrochemist said:

A Marcus said:

The math revolving around depth-of-field formulas is a subset of the formula used to find the hyperfocal distance. We multiply the working diameter of the camera’s lens (aperture diameter in millimeters) by 1000. Why 1000? The diameter of the aperture (f-number) viewed from 1000 diameters distance will be seen as a super tiny dimensionless point. Thus 1000x the dimeter works nicely to derive the hyperfocal distance. This value might not be stringent enough for giant enlargements. These are often based on 1/2000 of the dimeter of the iris. Kodak often used 1/1750 and Leica uses 1/1500 for critical work. Depth-of-field calculators often use different sizes. The choice is a guesstimate based on the magnification required to make the display image and the distance the observer is from this image.

Click to expand...

That's NOT the formula for determining the hyperfocal distance. It's a rule of thumb that might give a rough idea for a specific format.

To actually determine it you need to use the laws of optics & the circle of confusion appropriate for the sensors size & final output/viewing conditions.

Click to expand...

Found a proper formula for you :

hyperfocal distance = focal length + focal length^2 /( f number * circle of confusion)

The hyperfocal distance varies with focal length, the f-number and the permissible diameter of the circle of confusion. The circle size may be expressed as a fraction of the focal length. The hyperfocal distance is thus proportional to the focal length and inversely proportional to the f-number. Thus the hyperfocal distance is the effective aperture multiplied by that fraction of the focal length accepted as the standard for the diameter of the circle of confusion. Paraphrased "Photographic Lenses" by C. B. Neblette.

A Marcus said:

The hyperfocal distance varies with focal length, the f-number and the permissible diameter of the circle of confusion.

Click to expand...

All of those variable are taken account of in the equation I quoted, unlike in your initial rule of thumb.

A Marcus said:

The circle size may be expressed as a fraction of the focal length.

Click to expand...

If the circle referred to here is circle of confusion, it can be BUT it doesn't vary with focal length. It could equally be expressed in miles, light years or carbon-carbon bond lengths (all very unwieldy units but valid)

When shooting with a single camera, displaying uncropped images in the same way & viewing in the same way the CoC will stay the same, irrespective of the lens used.

A Marcus said:

The hyperfocal distance is thus proportional to the focal length and inversely proportional to the f-number.

Click to expand...

factorising the formula I gave previously & applying the definition of f/number allows the formula to be rewritten as

HF dist = FL (1+ 1/(aperture diameter * CoC))

So we can see it is indeed directly proportional to focal length provided we measure the actual iris opening & don't use f-numbers to measure the iris opening.

A Marcus said:

Thus the hyperfocal distance is the effective aperture multiplied by that fraction of the focal length accepted as the standard for the diameter of the circle of confusion. Paraphrased "Photographic Lenses" by C. B. Neblette.

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If you're going to quote sources DONT paraphrase them & change the meaning. I doubt your reference got it wrong (though it's possible)

Sorry I don't have the reference for where I got my formula I noted it down several years ago and it could have come form one of the multitude of books on optics I have collected (or possibly just from the net) No doubt it's somewhere in Rudolf Kingslake's 'Lenses in hotography' or 'applied optic & optical engineering vol 3' both recent favorite reads of mine.

My objective was to clarify why short focal length lenses have a greater span of depth-of-field for the same f-stop setting, then longer focal length lenses. Let’s read your explanation.

Keep in mind, there are many ways to skin a cat! By the way, there are many references that talk about the relationship, working iris diameter to hyperfocal/depth-of-field math. Also, I find it counterproductive to continue, all we do is confuse because we both know the answers however our valid answers differ.

Are you people really arguing about depth of field calculations? There are online calculators for that.

It’s the same for focal length, distance, magnification, and aperture diameter. These were all settled long ago.

Am I the only person here who took actual college classes in photography?

A Marcus said:

The hyperfocal distance varies with focal length, the f-number and the permissible diameter of the circle of confusion. The circle size may be expressed as a fraction of the focal length. The hyperfocal distance is thus proportional to the focal length and inversely proportional to the f-number. Thus the hyperfocal distance is the effective aperture multiplied by that fraction of the focal length accepted as the standard for the diameter of the circle of confusion. Paraphrased "Photographic Lenses" by C. B. Neblette.

Click to expand...

A Marcus said:

My objective was to clarify why short focal length lenses have a greater span of depth-of-field for the same f-stop setting, then longer focal length lenses. Let’s read your explanation.

Keep in mind, there are many ways to skin a cat! By the way, there are many references that talk about the relationship, working iris diameter to hyperfocal/depth-of-field math. Also, I find it counterproductive to continue, all we do is confuse because we both know the answers however our valid answers differ.

Click to expand...

FWIW I've found a source and old version of 'The Focal Encyclopedia of Photography' that gives your version of hyperfocal distance (probably quite reasonable for fixed lens roll film cameras, which were often just used for contact prints).

My reference goes on to give the formula for 'an absolute circle of confusion' as H=F²/cf

where H is HF dist, F is focal length, c is CoC & f is f/number which is the same as the initial formula I quoted except for the (pretty insignificant) lone F term (as F<<H).

ALL the formula quoted show shorter focal length lenses have shorter hyperfocal distances at any f-stop. None of them (including your rule of thumb) actually give DOF, but as hyperfocal distance is a function of DOF that's not an issue at least if it's explained.

I've used at least a 100 fold range of focal lengths on my DSLR (10mm fish eye to 1000mm ultra telephoto) but the CoC has remained fairly constant in all of these (I usually print to A4 but have been known to crop). Several of my mirrorless cameras have different CoC values when printing to the same size even when using exactly the same lenses.

I don't think the approximation CoC= ~focal length/1000 is valid anymore. But all the points in your objective are met by the more general formula.

petrochemist said:

A Marcus said:

The hyperfocal distance varies with focal length, the f-number and the permissible diameter of the circle of confusion.

Click to expand...

All of those variable are taken account of in the equation I quoted, unlike in your initial rule of thumb.

petrochemist said:

The circle size may be expressed as a fraction of the focal length.

Click to expand...

If the circle referred to here is circle of confusion, it can be BUT it doesn't vary with focal length. It could equally be expressed in miles, light years or carbon-carbon bond lengths (all very unwieldy units but valid)

When shooting with a single camera, displaying uncropped images in the same way & viewing in the same way the CoC will stay the same, irrespective of the lens used.

petrochemist said:

The hyperfocal distance is thus proportional to the focal length and inversely proportional to the f-number.

Click to expand...

factorising the formula I gave previously & applying the definition of f/number allows the formula to be rewritten as

HF dist = FL (1+ 1/(aperture diameter * CoC))

So we can see it is indeed directly proportional to focal length provided we measure the actual iris opening & don't use f-numbers to measure the iris opening.

petrochemist said:

Thus the hyperfocal distance is the effective aperture multiplied by that fraction of the focal length accepted as the standard for the diameter of the circle of confusion. Paraphrased "Photographic Lenses" by C. B. Neblette.

Click to expand...

If you're going to quote sources DONT paraphrase them & change the meaning. I doubt your reference got it wrong (though it's possible)

Sorry I don't have the reference for where I got my formula I noted it down several years ago and it could have come form one of the multitude of books on optics I have collected (or possibly just from the net) No doubt it's somewhere in Rudolf Kingslake's 'Lenses in hotography' or 'applied optic & optical engineering vol 3' both recent favorite reads of mine.

Click to expand...

Is anybody reading this stuff?

tEdolph

SmilerGrogan said:

Are you people really arguing about depth of field calculations? There are online calculators for that.

It’s the same for focal length, distance, magnification, and aperture diameter. These were all settled long ago.

Am I the only person here who took actual college classes in photography?

A Marcus said:

The hyperfocal distance varies with focal length, the f-number and the permissible diameter of the circle of confusion. The circle size may be expressed as a fraction of the focal length. The hyperfocal distance is thus proportional to the focal length and inversely proportional to the f-number. Thus the hyperfocal distance is the effective aperture multiplied by that fraction of the focal length accepted as the standard for the diameter of the circle of confusion. Paraphrased "Photographic Lenses" by C. B. Neblette.

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High school.

tEdolph

SmilerGrogan said:

Are you people really arguing about depth of field calculations? There are online calculators for that.

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There are indeed but some of them use a fixed CoC for all their calculations and thei're often not available where I'm shooting...

SmilerGrogan said:

It’s the same for focal length, distance, magnification, and aperture diameter. These were all settled long ago.

Am I the only person here who took actual college classes in photography?

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I'm sure you're not, but if the course you took was anything like my daughters it would be very little to do with photography - her course concentrated much more on playing with prints after the photography's done than actually photography itself!

There are also people here who design lenses, teach optics, etc. Some of them would almost certainly know much more than any of the lecturers of your course.

kiwi2 said:

Here is the post Bill is referring to...

www.dpreview.com/forums/post/63626863

"Yes a larger aperture lets more light in. But then you have to factor in another basic principle of optics. The more you magnify something the dimmer it becomes."

If you hold the aperture diameter constant and increase focal length, then brightness decreases. This basic principal is accepted with other optics like telescopes and microscopes and binoculars...

https://sciencing.com/happens-power-high-power-microscope-8313319.html

https://starizona.com/tutorial/understanding-magnification/

https://www.thespruce.com/binocular-magnification-386991

It also applies to camera optics as well.

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This is an incredibly photocentric viewpoint. Magnifying something does not make it dimmer, except possibly in the case of random posts on the internet, including this one.

Short of using such a massive lens that I blot out the very light illuminating the subject, I cannot, in any way, make a subject dimmer, ie, less illuminated. The surface reflection will remain constant, regardless of what aperture, focal length, or sensor quantum efficiency is used.

What will change is the amount of light gathered by your optics and medium.

Since you are reducing the volume of light (typically a cylinder with a diameter roughly matching your recording medium, whether it be film, cmos, ccd or silver-plated copper, and a "length" roughly akin to the exposure time) captured in order to produce a magnified view, it stands to reason resulting image would therefore be, "dimmer".

But the observer effect is not so pronounced in regular photography that I can affect the reflective properties of my subject by zooming in on them.

But if you possess such photographic equipment, please, do not darken my doorway, nor my frame.

**Disclaimer: What started as a pedantic tirade has hopefully transmogrified into a somewhat facetious, tongue-in-cheek, mis-interpretation of the parent post.

If they are all so educated why are they having so much trouble with basic photography? There is a lot of confusion between cause and effect, incident vs. reflected, and similar issues.

And I enjoy doing a little pencil and paper math as much as anyone, but do you really sit out in the field and do DOF equations? You can print out charts, you know.

petrochemist said:

SmilerGrogan said:

Are you people really arguing about depth of field calculations? There are online calculators for that.

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There are indeed but some of them use a fixed CoC for all their calculations and thei're often not available where I'm shooting...

petrochemist said:

It’s the same for focal length, distance, magnification, and aperture diameter. These were all settled long ago.

Am I the only person here who took actual college classes in photography?

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I'm sure you're not, but if the course you took was anything like my daughters it would be very little to do with photography - her course concentrated much more on playing with prints after the photography's done than actually photography itself!

There are also people here who design lenses, teach optics, etc. Some of them would almost certainly know much more than any of the lecturers of your course.

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Bill Ferris said:

almostgreen said:

You say: "The lens maintains a constant f/2.8 throughout the zoom range by increasing the aperture as focal length increases."

Is this increase in aperture to maintain the same f stop the reason why longer focal length lenses have less depth of field than shorter focal length lenses? If anyone can explain this clearly, Bill, it is you.

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In simplest terms, if you are a fixed distance from a subject, depth of field becomes more shallow as the lens entrance pupil increases.

If one frames a shot at 50mm f/8 and focuses on a subject 10-feet in front of you, the image will have a certain depth of field. From that same location using thay same gear, open the lens aperture to f/5.6 and make a photo of the same subject. Open up to F4, f/2.8 and make photos. Compare the images. The images made at the smaller f-stops (larger entrance pupils) will have noticeably shallower depths of field.

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Funny that aperture and focal length work in opposite ways. Wider aperture means worse depth of field but shorter focal length means better depth of field